The present invention relates to a spin coating process, and more particularly, to a spin coating process for providing a liquid coating of uniform thickness over the entire surface of a base plate.
In the manufacture of semiconductor devices, a spin coating process has heretofore been employed to coat the photoresist onto a base plate. For example, the base plate or substrate may be a photomask blank, silicon wafer or the like constructed as a glass sheet having a metal film. Both a manual process for coating photoresist liquids and an automatic coating process using an automatic photoresist coating machine are known. In the manual coating process, the base plate is placed on a turntable of a spinner, coating liquids are taken by hand into, for example, a pipette, the liquids are dropped on a substantially central portion of the base plate, and thereafter, the turntable is rotated. When the coating liquids from the pipette are initially dropped on the base plate, the coating liquids are spread only onto the central portion. Thus, after the coating liquids have been fed from the pipette to the central portion of the base plate, the turntable is rotated at a low speed to spread the coating liquids over the entire surface of the base plate by means of a centrifugal force after which the table is rotated at a high speed to spread extra liquid so as to obtain the desired thickness of film.
In the automatic coating machine, a nozzle for feeding coating liquids is arranged above the turntable. when the base plate is placed on the turntable, a given quantity of coating liquids are fed to a central portion of the base plate. Thereafter, the turntable is rotated at a low speed, and the coating liquids are spread over the entire surface of the base plate. Subsequently, the turntable is rotated at a high speed to obtain a coated film of desired thickness.
Where the size of the base plate is small, for example, from 3 to 4 inches, and/or certain kinds of photoresist coating liquids are used, the abovementioned processes will suffice. However, where the size of base plate is large such as 5 or 6 inches or certain other kinds of coating liquids are used, it has been found that even for smaller sizes of base plates, producing a coating of uniform thickness over the entire surface is impossible utilizing the abovementioned processes. For example, where the coating liquids are silver halide emulsions of which the binder is gelatin or where the coating solvent has a quick-drying property such as acetone, even if the coating liquids are fed from the nozzle to the central portion after which the turntable is rotated at a low speed to spread the coating liquids over the entire surface and the turntable is further rotated to obtain the desired film thickness, it nonetheless still results that the film thickness of a portion to which the coating liquids are first applied (particularly, the film thickness in a boundary in that portion) is clearly different from that of a portion spread by rotation of the turntable. The phenomenon is particularly remarkable where silver halide emulsions are coated onto the base plate. When the film thickness after coating different in parts as described above, the sensitivity and developing speed in the differenting portions are different as a consequence of which the line width of a developed image will vary which is a most certain disadvantage. For example, the electrical characteristics obtained by forming a semiconductor device using such an image will be different for different parts of the device.
This problem will be described in more detail with reference to FIG. 1, which illustrates a conventional spin coating process. When a base plate 10 or substrate is placed on a turntable (not shown), coating liquids are dropped thereon from a nozzle 11 above the central portion of the base plate and the coating liquids are spread over the central portion 12 of the base plate forming a boundary 13. Then, when the turntable is rotated at a low speed (normally at 100 to 1000 rpm), the coating liquids are spread outwardly by centrifugal force to cover the whole surface of the base plate. Thereafter, when the turntable is rotated at a high speed (normally at 2000 to 6000 rpm) for a predetermined period of time, extra coating liquids are removed by the centrifugal force to obtain a film of uniform thickness as desired. Some photoresist coating liquids may be coated substantially uniformly in accordance with this process. However, if an attempt is made to coat some kinds of photoresist coating liquids such as a quick-drying solvent like acetone used as the coating solvent or gelatinous silver halide emulsions in accordance using such a process, a coating of uniform thickness over the whole surface may not be obtained. For example, when the turntable is rotated at a low speed after the gelatinous silver halide emulsions have been fed as shown in FIG. 1, the emulsions on the base plate are not uniformly spread outwardly but are spread along several radial passages 20 as shown in FIG. 2. As a consequence, the emulsions are not coated onto portions of the base plate where no radial passages 20 are formed. It is believed that such a phenomenon occurs because of poor wetting of the silver halide emulsions on the base plate. Since many photoresist coating liquids are dissolved in an organic coating solvent, the base plate is well wetted due to the organic solvent and such a phenomenon generally does not occur. However, if the coating solvent has an extremely high volatility, a boundary mark 13 as in FIG. 1 often appears on the film after coating.
In order to overcome the above-described difficulty, the inventors have earlier proposed, as disclosed in Japanes Laid-Open Patent Application No. 49955/1978, a spin coating method in which, after a coating solution is supplied onto a substrate held horizontally, the substrate is rotated around a rotary shaft perpendicular to the substrate so as to obtain a coated film on the substrate. In this process, it is not until after the coating liquid is substantially uniformly coated substantially over the entire surface of the substrate that the substrate is rotated.
According to this method, the coating liquid is substantially uniformly coated substantially over the entire surface of the substrate before the substrate is rotated, as described above. Therefore, with any kind of coating solution, the entire surface of a substrate can be coated to a uniform thickness even if the size of the substrate is large. This is one of the significant effects of this method. Especially in the case where a substrate is coated with the abovedescribed silver halide emulsion, the coated substrate is free from the fatal defect that the emulsion is spread outwardly along radial paths on the substrate. That is, the substrate can be uniformly coated with the emulsion.
However, this method has still remaining problems. That is, the method is disadvantageous in that when a coating device such as a bar coater is employed as means for supplying a coating solution uniformly over the entire surface of a substrate before the substrate is rotated, the coating device must be cleaned for every coating operation. In order to apply such a coating method to an automatic coating machine, in addition to the coating device a cleaning device must be provided, and accordingly the entire device becomes intricate in construction and high in manufacturing cost. Instead of the above-described coating devices, a coating solution supplying nozzle may be employed to supply the coating solution to the substrate. In this case, the nozzle is operated in a scanning mode to a supply the coating solution substantially over the entire surface of the substrate. However, this method is still disadvantageous in that it is necessary to supply an amount of coating solution larger than that in the case where a coating device is used as mentioned above. Furthermore, the method is not economical because a larger part of the coating solution supplied onto the substrate is thrown off the substrate by the subsequent rotation and often the coating solution thus thrown off cannot be used again.